The present invention relates to heat exchangers, and more particularly to heat exchangers having tubes for transferring heat between a fluid flowing through the tubes and another fluid in contact with the exterior of the tubing. Specifically, the present invention relates to a method of making heat transfer tubing for use in a heat exchanger of the type wherein a fluid to be cooled is passed through the tubing and a boiling liquid is in contact with the exterior of the tubing whereby heat is transferred from the fluid in the tubing to the boiling liquid.
In an evaporator of certain refrigeration systems a fluid to be cooled is passed through heat transfer tubing while refrigerant in contact with the exterior of the tubing changes state from a liquid to a vapor absorbing heat from the fluid within the tubing. The external and internal configuration of the tubing is important in determining the overall heat transfer characteristics of the tubing. For example, it is known that the presence of vapor entrapment sites on the external surface of a tube enhance the transfer of heat from the fluid within the tube to the boiling refrigerant surrounding the tube. It is theorized that the provision of vapor entrapment sites creates sites for nucleate boiling. According to this theory the trapped vapor at or slightly above the saturation temperature increases in volume as heat is added until surface tension is overcome and a vapor bubble breaks free from the heat transfer surface. As the vapor bubble leaves the heat transfer surface, liquid refrigerant enters the vacated volume trapping the remaining vapor and another vapor bubble is formed. The continual bubble formation together with the convection effect of the bubbles traveling through and mixing the liquid refrigerant results in improved heat transfer.
A nucleation site is most stable when it is of the re-entrant type. See, for example, Griffith, P. and Wallis, J. D., "The Role of Surface Conditions in Nucleate Boiling", Chemical Engineering Progress Symposium Series, No. 30, Volume 56, pages 49 through 63, 1960. In this context a re-entrant nucleation site is defined as a cavity in which the size of the surface opening is smaller than the subsurface cavity. U.S. Pat. Nos. 3,696,861 and 3,768,290 disclose heat transfer tubes having such re-entrant type cavities.
Also, it is known that an excessive influx of ambient liquid can flood or deactivate a nucleation site. See, for example, Bankoff, S. G., "Entrapment of Gas in the Spreading of a Liquid Over a Rough Surface", A. I. Ch. E. Journal, Volume 4, pages 24 through 26, March, 1958. In this regard, it is known that a heat transfer surface having "minute tunnels" communicating with the surroundings through openings having a specified "opening ratio" may provide good heat transfer. See, for example, U.S. Pat. No. 4,060,125 to Fujie, et al.
In regard to the interior surface configuration of a heat transfer tube it is known that providing an internal ridge on the tube may enhance the heat transfer characteristics of the tube due to the increased turbulence of the fluid flowing through the ridged tube. See, for example, U.S. Pat. Nos. 4,059,147 and 3,881,342. These patents relate to heat transfer tubes having exterior re-entrant type nucleation cavities and having interior ridges.
As disclosed in U.S. Pat. Nos. 4,059,147 and 3,881,342, a heat transfer tube may be formed by rolling a tool arbor, having discs attached thereto, over the external surface of the unformed tube to form fins on the exterior of the tube, while, at the same time, pressing the tube against a grooved mandrel to form interior ridges on the tube. The external fins are bent over to form cavities by drawing the tube through a die after the fins are formed. All the cavities have continuous openings to allow fluid communication with the surroundings of the tube and the configuration of the cavities is critical in achieving optimal heat transfer characteristics with such a tube.
A heat exchanger, such as an evaporator of a refrigeration system, utilizing high performance heat exchange tubing, such as tubing having the features described previously, has increased capacity over that capacity obtained when the heat exchanger is constructed using other types of tubing, such as conventional straight-finned tubing. However, a heat exchanger constructed with high performance tubing is cost-effective only if any increase in the cost of manufacturing the high performance tubing is offset by the improved capacity and/or the reduced size of the heat exchanger. Therefore, heat transfer tubing having performance advantages, such as exterior re-entrant nucleation cavities and interior ridges, without performance disadvantages, such as "flooding" exterior nucleation cavities, is desirable from the viewpoint of better performance resulting in improved cost-effectiveness. Also, the more efficiently such a high performance heat transfer tube is constructed the more cost-effective is the tube.